1. Field of the Invention
The present invention relates to a method for controlling a refrigeration system utilizing a thermostatic expansion valve, and the construction of the thermostatic expansion valve best suited for the method.
2. Description of the Related Art
In a refrigeration system, there is a thermostatic expansion valve which controls the flow rate of a refrigerant flowing into an evaporator, so as to keep constant the degree of superheat of the refrigerant, i.e., the difference between the evaporating temperature of the refrigerant in the evaporator and the temperature of superheated steam of the refrigerant in the evaporator after heat is exchanged with outside air. The expansion valve functions also as a refrigerant-pressure reducer.
The thermostatic expansion valve is opened to make the degree of superheat positive. In such a situation, the evaporator is used as a dry-type evaporator. This type will prevent the liquid from flowing back to a compressor.
Recently, new compressors which can vary their capacity in response to heat load were commercialized, but traditional ones which cannot fully control their capacity have been still widely used. In the traditional ones, the capacity thereof sometimes will rise to such an extent that there is a large excess over a value being necessary and sufficient to cause the refrigeration system to remove a heat load. This excess causes the evaporating pressure and evaporating temperature of the refrigerant in the evaporator to be lowered. In such a case, the outer surface of the evaporator is frosted and prevented from effecting satisfactory heat exchange, so that the refrigeration capacity of the refrigeration system is considerably reduced.
Such a situation often happens to those air conditioners which are carried by compact passenger cars. In these compact cars, a compressor is driven by a rotary force transmitted from an engine through a clutch. Therefore, the capacity of the compressor increases during high-speed operation of the engine, without regard to the value of the heat load. Thus, the air conditioners obtain an unnecessarily large capacity, thereby causing the aforesaid unsatisfactory situation.
In U.S. Pat. No. 4,428,718, for example, there is disclosed a method for controlling the capacity of a compressor in order to eliminate the aforementioned unfavorable situation. However, this conventional control method for the compressor capacity cannot be fully effective in comparison with its high cost performance. Conventionally, therefore, various other methods to solve this problem have been proposed, in which various other elements of the refrigeration system are controlled without controlling the compressor capacity.
In one particularly simple method, among these conventional methods, a part of the refrigerant flowing toward the evaporator, under the flow-rate control of the thermostatic expansion valve, is returned to the compressor through a by-pass circuit, without passing through the evaporator.
In the by-pass circuit disclosed in Japanese Utility Model Disclosure No. 61-153875, a constant-pressure expansion valve is mounted on the by-pass circuit. The constant-pressure expansion valve opens without regard to a control signal for the thermostatic expansion valve when the evaporating pressure of the refrigerant in the evaporator is reduced to a predetermined level or below.
The thermostatic expansion valve and the constant-pressure expansion valve described in the above Japanese document are combined to make one unit, as shown in FIG. 1. The unit includes a first drive section I, which is controlled by temperature and functions as the thermostatic expansion valve, and a second drive section II, which is controlled by pressure and functions as the constant-pressure expansion valve. Section I is constructed of a thermo-tube 10, a working fluid circulation pipe (capillary tube) 12, an operating chamber 14, and a first diaphragm 16, while section II is constructed of a constant-pressure chamber 18 and a second diaphragm 20. In this arrangement, when the capacity of a compressor rises so that the evaporating pressure and the evaporating temperature of the refrigerant in an evaporator are lowered, a refrigerant passage in the unit is used as a by-pass while the thermostatic expansion valve operates to close the passage. This combined unit, which has two functions as two kinds of valves, makes its whole appearance compact.
When the compressor operates in a predetermined range of its capacity so that the evaporating pressure and evaporating temperature of the refrigerant in the evaporator are relatively high, the pressure of a working fluid in the first drive section I becomes higher than that of a constant-pressure fluid in the constant-pressure chamber 18. The former pressure is transmitted to the upper surface of the second diaphragm 20 by a force-transmission member 22, which is interposed between first the and second diaphragms 16 and 20. The second diaphragm 20 causes a valve body 26 to move to its open position so that the transmitted pressure balances the sum of the urging force of a bias spring 24 and the pressure of the refrigerant from a condenser, applied to the lower surface of the second diaphragm 20. However, when the compressor begins to be operated over the predetermined range of its capacity so that the evaporating pressure and the evaporating temperature of the refrigerant in the evaporator drops, the pressure of the working fluid in the first drive section I becomes lower than the pressure of the constant-pressure fluid sealed in the constant-pressure chamber 18 of the second drive section II. Consequently, since the sum of the pressure of the working fluid in the first drive section I and the constant pressure in the second drive section II has been larger than the sum of the pressure of the refrigerant in the refrigerant passage of the unit and the biasing force of the bias spring 24, the valve body 26 moves toward its open position until the force applied on the lower surface of the second diaphragm 20 (the latter sum) balances the force applied on the upper surface of the second diaphragm 20 (the former sum).
FIG. 2 shows a gradually curved first line, which is schematically made by adding a biasing force of the bias spring 24 to the pressure-evaporating temperature chart of the refrigerant R-12 in the evaporator, and a lower partially bent, second line, which is schematically made by adding the constant pressure in the constant-pressure chamber 18 to the pressure-evaporating temperature chart of the working fluid in the first drive section I. As seen from FIG. 2, the valve opens without regard to the degree of superheat when the evaporating temperature of the refrigerant is lower than a predetermined level, in which the force applied to the upper surface of the second diaphragm 20 is larger than the force applied to the lower surface of the second diaphragm 20. Therefore, the capacity of the evaporator is reduced to such an extent that a part of the refrigerant remains in a liquid phase even at the outlet of the evaporator, so that the outer surface of the evaporator is prevented from becoming frosted. Liquid flowing back to the compressor is very harmful if the compressor is a reciprocating-type, because such liquid back flow may cause a valve in the compressor to be broken. In this case, however, such a liquid back flow is not harmful to the compressor, since the compact refrigeration system, using the combined constant-pressure/thermostatic expansion valve, usually uses a rotary-type compressor having a relatively low compression ratio.
Although the refrigeration system described above is theoretically effective, it has the following various structural problems.
A first problem is that the force transmission member 22 should be mounted between the first and the second diaphragms 16 and 20 of the first and the second drive sections I and II.
In order to transmit accurately a displacement of the first diaphragm 16 to the second diaphragm 20, the force transmission member 22 must smoothly slide on the inner peripheral surface of the constant-pressure chamber 18 of the second drive section II. Furthermore, in order to downwardly and uniformly press the upper surface of the second diaphragm 20, the force transmission member 22 must be shaped so that its contact surface on the second diaphragm 20 is wide enough, as shown in FIG. 1. For these reasons, the force transmission member 22, having a complicated configuration, must be worked with high accuracy.
A second problem is that the space in the constant-pressure chamber 18 of the second drive section II for the sealed fluid must be so large that a change of its volume due to a transformation of the first diaphragm 16 can be neglected. This necessity leads to two contradictory requirements, i.e. that the space for the working fluid in the first drive section I be minimized, and that the diameter of the first diaphragm 16 be maximized in order to produce a sufficiently great force by of a small amount of the working fluid.
A third problem is that the use of the two diaphragms 16 and 20 requires two welding processes in the manufacturing operation, thus entailing increased manufacturing costs, a higher possibility of malfunction, and a hindrance to compact designing.